The death of retinal photoreceptor cells is one of the primary causes of vision loss. Substantial loss of photoreceptors occurs with age in normal human eyes, and is significantly exacerbated in Age-related Macular Degeneration (AMD), a leading cause of blindness in the United States. Photoreceptor cell death is also the cause of vision loss in several inherited retinal degeneration diseases, such as Retinitis Pigmentosa. Robust metabolic activity is necessary for cells to support their physiological functions and maintain their viability. Metabolic defects that result in deterioration of metabolic competence underlie a variety of degenerative diseases including retinal degenerations. Thus, treatments that protect or enhance the metabolic competence of photoreceptors may have significant therapeutic potential. Currently, there is no assay for testing the beneficial or possibly deleterious effects of potential treatments on human photoreceptor cells. We propose to establish a novel assay for measuring the metabolic competence of human photoreceptors. The assay uses single living rod photoreceptors isolated from defined regions of the human retina. The eyes and retinas will be clinically evaluated and rod photoreceptor cells will be isolated from retinas free of disease. Metabolic competence will be measured from the conversion of supplied all-trans retinal to retinol, a reaction that requires NADPH and the cell is equipped to carry out as part of its light-detecting function. The capacity of a cell to generate NADPH will be estimated from the fraction of all-trans retinal converted to retinol, which can be directly measured with fluorescence imaging from the intrinsic retinoid fluorescence. The proposed experiments will provide the first measurements of human rod photoreceptor metabolism and its variation across the retina. They will also establish the baseline parameters for the future use of the assay in the evaluation of proposed treatments for retinal degenerations. The aims of the research are: Specific Aim #1: Determine the dependence of the metabolic competence of human rod photoreceptors on the type and concentration of metabolic substrate. Specific Aim #2: Determine the spatial dependence of the metabolic competence of human rod photoreceptors across the retina. The research will establish an assay for testing the efficacy of therapeutic approaches at the single human photoreceptor cell level. It will determine the relation between the clinical assessment of a retina free of disease and the metabolic competence of individual photoreceptor cells isolated from that retina. The assay will be a valuable tool in the development of treatments that aim to improve human photoreceptor metabolic competence and thereby extend their lifespan. Such information will play a key role for the treatment of retinal degeneration diseases including AMD and Retinitis Pigmentosa.